The present invention relates to a method for measuring the degree of alloying of galvannealed steel sheets and more particularly to a method of a non-destructive, continuous and qualitative measurement of degree of alloying of galvannealed steel sheets through the X-ray diffraction technique.
For the purpose of improving the characteristics of hot-dip galvannealed steel sheets to painting, paint adhesion and welding, a great deal of research work has been conducted on the nature of galvannealed steel sheets in which, through application of heating before solidification of the zinc surface layer just after galvanizing, mainly .delta..sub.1 -and/or .zeta.-phase is grown in the galvanized layer.
As a result of the research work, it is now known that the qualitative characteristics of the galvannealed steel sheets is greatly dependent upon the degree of alloying, i.e. the degree of mutual diffusion between iron and zinc.
When the application of heat to the coating just after the hot-dip galvanizing is insufficient, the .eta.-phase remains in the surface layer and this leads to poor paintability, paint adhesion and welding. Whereas, when the heat application is excessive, overfull diffusion of iron into the zinc coated layer occurs and this lowers fitness of the coated layer to other treatments and resistance of the plated layer against corrosion. Therefore, in order to produce galvannealed steel sheets with excellent quality, it is indispensable to properly control the galvannealing process on the basis of a continuous measurement of the degree of alloying and to keep it within a prescribe range.
In case galvannealed steel sheets are produced by the continuous hot-dip galvanizing process, the degree of alloying is dependent upon various factors such as the thickness of the galvanized steel sheet, the quantity of the coated zinc, the composition of the base steel, the change in the composition of the zinc bath, particularly the change in the aluminum concentration, the heating rate, the maximum heating temperature, the evenness in heating at that temperature, the cooling rate and the variation in the heating atmosphere.
For example, it is required to employ a high heating temperature and a long treatment period when the thickness of the galvanized steel sheet and the quantity of the coating are large. In this connection, however, it is extremely difficult to produce galvannealed steel sheets with proper degree of alloying by adjusting the temperature of the heating furnace and the processing speed only by visual check, as degree of alloying is dependent not only upon the heating temperaturre and period but also upon other factors which influence each other in very complicated ways.
In order to measure the degree of alloying of galvannealed steel sheets, a method has been conventionally and to a major degree employed to detect the change of surface colour tone after the heat application by a direct optical observation or to determine the degree of alloying on the basis of the change of colour tone detected by a photometer. In this measurement system, it is very difficult even for a skilled operator to distinguish delicate and slight change in the colour tone exactly and this difficulty in the practical measurement often causes production of galvannealed steel sheets of somewhat low quality. This is also the case when measuring the degree of alloying by the photometer.
Even in case of products processed under the same heating conditions, reflexibility of light from the surface of galvannealed steel sheets fluctuates from product to product depending upon the type of the base steel, the variation in the zinc bath composition, the heating atmosphere and presence of stains on the surface of galvannealed steel sheets. Such a fluctuation in the reflexibility of light leads to lowered reliability of the measurement.
In addition, no available direct information as to the state of the iron-zinc intermetallic compound in the galvannealed layer can be obtained through the measurement by the direct optical observation of the colour tone of the galvannealed surface and the measurement by detection of the reflexibility of light. Thereby in the case of these measurement systems used for evaluation of the degree of alloying of the galvannealed steel sheets, criteria need to be changed in accordance with variation in the quantity of the coating. Thus, the measurement systems based on the colour tone and the reflexibility of light are not recommended for use in connection with a practical continuous galvannealing process.
For example, a galvannealed steel sheet of good quality may be obtained with the reflexibility ranging from 30 to 35% when the quantity of the coating on one side is 60gr/m.sup.2. However, the same range of the reflexibility does not always assure the production of a galvannealed steel sheet with the same good quality when the quantity of the coated zinc is 90gr/m.sup.2. At this range of the reflexibility with the above-mentioned quantity of the coated zinc, the degree of alloying is too high, and thereby the ductility of the coating and the resistance against corrosion are seriously degraded.
Thus, it will be readily understood that it is extremely difficult to successfully avoid production of galvannealed steel sheets of degraded quality in the continuous hot-dip galvanizing and galvannealing process when the degree of alloying is measured by the conventional optical observation of the colour tone and the photometer detection of the reflexibility. Particularly, in consideration of the recent trend in which there is a strong demand for galvannealed steel sheets with a large quantity of coated zinc, e.g. a galvannealed steel sheet of a quantity of coated zinc exceeding 120gr/m.sup.2 on one side, the conventional systems for measuring the degree of alloying being unsatisfactory to minimize unacceptable production of galvannealed steel sheets with unacceptablly degraded quality.